1. Field of the Invention
The present invention relates to a recirculating sheet feeding apparatus embodied preferably in a recirculating automatic document feeder, which is used in a copying machine and the like for feeding documents seriatim to an exposure station from a document table onto which a plurality of document sheets are stacked, and returning the documents from the exposure station to the document table after exposure.
2. Description of the Prior Art
Recently, in an electrostatic copying machine, it has been known to install a document feeder which transports a stack of documents seriatim to a reading station for optical scanning of a plurality of equal-sizes document sheets.
In such a document feeder in the prior art for feeding the documents stacked on a document table to a transporting path wherein the reading station is formed, air is blown against the edge faces of the documents from the downstream side in the feeding direction. Thereby, the edge faces of the documents on the downstream side in the feeding direction are loosened.
In this state, by vacuum sucking means and an endless feeding belt provided above the document table, the document on top of the plurality of document sheets is sucked onto the peripheral surface of the feeding belt so as to be fed to the transporting path.
In the document feeder constructed as described above, when the material or quantity of stacked document sheets to be fed varies, feeding conditions between feeding means changes, thus the document edges on the downstream side in the feeding direction are loosened insufficiently. As a result, sometimes a plurality of document sheets are sent as a single sheet of a document, and so-called multiple feeding takes place. Also, when the document is sucked onto the feeding belt by an insufficient vacuum sucking means, a poor feeding such as clogging may occur at feeding from the document table.
In some copying machines, a recirculating automatic document feeder which sends out the document seriatim from top of a stack of documents on a document table, and returns it to the bottom of the document table after the document has been exposed at an exposure station is installed. As disclosed, for example, in U.S. Pat. Nos. 4,413,901 and 4,368,973, in the conventional recirculating automatic document feeder, it was a common practice that a stack of documents on the document table were lifted and replaced after the document was returned and fed thereunder.
However, in the case wherein the document is returned in the manner aforementioned, it is complicated to control up-and-down movement of the stack of documents, causing the feeder itself to become larger. Besides, when the documents are moved up and down, their sides tend to become misaligned and corners to be turned up or down.
Although though it may be considered to force the document into the bottom of the stack of documents with a roller utilizing a stiffness of the document, in this case, since the document stops when its rear end has passed the roller, in order to return the document to a given position, a plurality of rollers or a transporting belt must be provided on the document table so as to keep pushing until the document is returned reliably to the given position. However, in such a case, the document may be stained by the duration of contact with the rollers or the transporting belt.